Multi-beam antennas may be used to reduce the number of antennas on a cellular base station tower. For example, a dual beam antenna is a type of multi-beam antenna that has separate inputs for two beams to be generated, an array of radiating elements, and a beam forming network that applies predetermined and opposite phase shifts to the beam inputs such that the beams are steered off antenna boresight in opposite directions.
One common problem in multi beam antennas is the port to port coupling between the beams that point equally away from the antenna boresight. This is a result of a transmit RF signal of one beam being reflected at the radiating elements, and the beam-forming network coupling the reflected signal through the receive path of a second beam. A high level of coupling between two beams can cause interference and/or damage to the receiver if one beam is transmitting while the other beam is receiving. To avoid this scenario, beam to beam isolation level is specified by an operator. Radiating elements in a multi-beam antenna are generally designed to radiate at a high efficiency to minimize the beam to beam coupling. Even then, certain amount of power from one beam can reflect to the other beam.